As mobile communication devices and mobile electronic devices have become widely used recently, demands for power sources usable for these devices are significantly increasing. Lithium secondary cells are in wide use as power sources for these devices owing to the larger electromotive force, high energy density and reusability thereof.
In order to improve the ease of carrying around the mobile communication devices and mobile electronic devices, lithium secondary cells which provide larger outputs and are more lightweight are needed, and studies are now conducted to find electrode materials fulfilling such needs. In order to decrease the weight of the lithium secondary cells, it is considered to use an organic compound as an electrode material because the specific gravity of an organic compound is about 1 g/cm3 and is smaller than that of lithium cobalt oxide, which is currently used as a positive electrode active material.
In a charged state of a lithium secondary cell, lithium ions are incorporated into a negative electrode to reduce the lithium ions. By contrast, in a positive electrode of the lithium secondary cell, a positive electrode active material is oxidized. In this step, depending on the oxidation state of the positive electrode active material, lithium ions may be separated from the positive electrode, or anions are incorporated into the positive electrode. In a discharged state of the lithium secondary cell, lithium in the negative electrode is oxidized to separate the lithium ions from the negative electrode. In the positive electrode, along with the reduction of the positive electrode active material, lithium ions may be incorporated into the positive electrode, or anions may be separated from the positive electrode.
For example, Patent Document 1 proposes using, as a positive electrode active material, a conductive polymer containing a quinone-based functional group having an oxidation/reduction activity in a structure thereof. Patent Document 2 proposes using, as a positive electrode active material, an organic compound having a disulfide group. Patent Document 3 proposes using, as a positive electrode active material or an electrode material, a polymer of 2,5-pyridinediyl, which is a pyridine compound having one carbon of a benzene ring substituted with nitrogen.
Where an organic compound having a disulfide group in a molecule thereof or a polymer compound of pyridinediyl mentioned above is used as a positive electrode active material of a lithium secondary cell, such a compound is reduced in a discharged state and thus lithium ions are coordinated. The lithium ions are provided from the negative electrode. In the case where the negative electrode is formed of a material which does not contain lithium ions, lithium ions need to be incorporated into the positive electrode or the negative electrode in advance.
Patent Document 4 proposes using an organic compound having a fulvalene structure as a positive electrode active material. Patent Document 5 proposes using an organic compound having a radical group as an active material of a secondary cell.
In the case where the organic compound disclosed in Patent Document 4 is used as positive electrode active material, the organic compound is oxidized in a charged state and anions having a negative electric charge are bonded or coordinated as counter ions. For example, in the case where lithium hexafluorophosphate is used as an electrolyte salt, the reaction in the positive electrode is represented by expression (1) below and the reaction in the negative electrode is represented by expression (2) below. In the expressions, R is the organic compound disclosed in Patent Document 4, and R* is a negative electrode active material.R+PF6−→[R+PF6−]+e−  (1)R*+Li++e−→[R*−Li+]  (2)
As is understood from the above, the reactions in the electrodes involve counter ions as well as the active materials. In addition, both anions and cations respectively move to the positive electrode and the negative electrode to cause electrode reactions. This indicates that the capacity of the secondary cell depends on the amount of the electrode salt such as lithium ions or anions as well as the mass of the active materials in the electrodes.
Especially in the case where a non-aqueous solvent is used for an electrolyte solution, the solubility of lithium ions or anions is about 1 to 2 M. Even if the energy density is desired to be increased, there is a limitation on the concentration at which such an electrolyte is dissolved in the solvent. Because of such a situation, it is considered to produce a secondary cell in the state where such ions are incorporated into the positive electrode or the negative electrode in advance, in order to increase the ion concentration in the cell.
In the case where graphite or aluminum is used for the negative electrode, lithium ions incorporated by the first cycle of charge partially remain incorporated into the negative electrode and form an initial irreversible capacity referred to as “retention”. Such a part of lithium ions is not involved in the reactions caused by charge/discharge. As a result, the lithium ion concentration in the cell is decreased. At this point, anions of the same electric charge level are incorporated into the positive electrode. In order to compensate for the ions which become non-involved in the reactions, it is considered to produce a secondary cell in the state where such ions are incorporated into the positive electrode or the negative electrode in advance.
As described above, it has been proposed to produce a secondary cell in the state where lithium ions or anions are incorporated into the positive electrode or the negative electrode in advance, in order to incorporate lithium ions into the positive electrode or the negative electrode or in order to compensate for the ions which become non-involved in the reactions. Methods for producing the secondary cell in this manner are roughly classified into three: (1) method using a chemical reaction, (2) physical method, and (3) electrochemical method.
(1) Method using a chemical reaction: Patent Document 6 discloses using a chemical reaction to incorporate ions into an electrode material. For this, an irreversible chemical reaction is used. Specifically, lithium acetylide-ethylene diamine complex and LiCoO2 or the like which is a positive electrode active material are put into contact with each other, thereby reducing, mainly, Co of LiCoO2 and thus incorporating Li ions in the lithium acetylide-ethylene diamine complex into LiCoO2.
(2) Physical method: Patent Document 7 discloses pasting a material to be used as counter ions directly to an electrode to cause a physical contact, thereby incorporating ions thereinto. Specifically, it is disclosed that lithium metal is put into direct contact with a negative electrode formed of graphite.
(3) Electrochemical method: Patent Document 8 discloses putting a positive electrode into an electrolyte solution together with a counter electrode and flowing an electric current between the positive electrode and the counter electrode using an external power source, thereby reducing the positive electrode and thus incorporating lithium ions into the positive electrode. The positive electrode having the lithium ions incorporated thereinto is built into a secondary cell together with a negative electrode, and thus the secondary cell is produced.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-154512
Patent Document 2: Japanese Laid-Open Patent Publication No. 05-074459
Patent Document 3: Japanese Laid-Open Patent Publication No. 1-178517
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-111374
Patent Document 5: Japanese Laid-Open Patent Publication No. 2002-151084
Patent Document 6: Japanese Laid-Open Patent Publication No. 5-54887
Patent Document 7: Japanese Laid-Open Patent Publication No. 11-86847
Patent Document 8: Japanese Laid-Open Patent Publication No. 11-31531